This project will support the establishment of my independent laboratory and its goals of leveraging basic discoveries on the molecular basis of heart development into improved diagnostics and therapies for heart disease. During the K99 portion of this award, I pursued postdoctoral training at Stanford University towards my transition to independency by: coursework in lab management, studies and laboratory research in cardiovascular biology, presentation of results, and publication of manuscripts. These efforts culminated in my acceptance of a tenure-track faculty position at the University of Oregon. There, my lab's research will focus on our discovery that endocardial BAF chromatin remodeling complexes have remarkably specific roles in two regions of the developing mammalian heart. In the ventricles, the BAF complex specifies the extracellular matrix required for morphogenesis of muscle cells into trabeculae by repressing transcription of a matrix protease, ADAMTSl. This regulation appears to be dynamic, as later in development ADAMTSl expression increases to prevent excessive trabeculation. At the endocardial cushions that develop into valves, the BAF complex regulates an endocardial-to-mesench3mtial transformation (EMT) that gives rise to cushion-populating cells. We hypothesize that endocardial BAF complexes establish regulatory "switches" at key loci to control developmental events in different regions of the heart. In the ventricles, we propose the BAF complex is recruited to ADAMTSl by specific cooperating factors to induce d3mamic changes in nucleosome organization to repress transcription. In the cushions, we hypothesize the BAF complex regulates transcription of secreted regulators of Wnt signaling. Misexpression of these factors in the absence of the BAF complex induces a premature and ectopic activation of Wnt that may block EMT. These hypotheses will be pursued using two Specific Aims: 1) Describe cis- and transacting factors that recruit the BAF complex to ADAMTSl. Delineate nucleosome modifications that cooperate with the BAF complex to repress ADAMTSl. 2) Determine if inhibiting Wnt signaling restores EMT in embryos lacking endocardial BAF complexes. Describe the expression of Wnt regulating transcripts as potential targets of the BAF complex in endocardial cushions.